Hi...
Coming a bit late..
As you see, with -Os, this produces:
/* THIS FIELD IS ONLY 16 BITS */
movl 4(%ebx), %eax
movw %ax, (%esi)
While the no-optimizations code looks like this:
movl 8(%ebp), %eax
movzwl 4(%eax), %eax
movl %eax, %edx
movl 12(%ebp), %eax
movw %dx, (%eax)
The first one is clearly loading a 32-bit value from an address that
may not have more than 16 bits written to it.
So, is it legal for the compiler to do that?
Interesting to look at this fact (never realize it before). Maybe in
-Os, compiler (gcc in this case?) assumes there is padding, either
inside the struct or between this struct and next variable/struct. Thus,
treating them as 32 bit is fine since in most cases you get zeroed page
when doing memory allocation. Clear advantage here, smaller code.
What if you put "packed" attribute at the the struct declaration? Still
see the same thing?
regards,
Mulyadi
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